Certain new types of telephone instruments provide a variety of functions in addition to conventional functions in telephone service. In order to provide these new functions, these instruments obtain power from a local source of alternating current. In order of maintain telephone service in the event that the source becomes inoperative, it is necessary for the instrument to provide its own direct current power while disabling the equipment used in providing the additional functions. Some of this equipment contains stored information which cannot be erased when the source becomes inoperative and erasure will ensue if such direct current power is not available.
The present invention is directed toward power supply control apparatus which can be used in the new types of instruments to provide the protective actions described above.